1. Field of the Invention
The present invention relates to electronic and microelectronic devices. In particular, the present invention relates to a plurality of materials showing a switching phenomenon and a built-in memory by the aid of which a new principle of storing and reading information in memory cells of semiconductor chips and a plurality of fundamental improvements on electronic or microelectronic devices can be achieved.
2. Description and Disadvantages of Prior Art
Although the present invention is applicable in a broad variety of microelectronic or electronic applications it will be described with the focus put on an application to memory cells as RAM (Random Access Memory), for example.
The need to remain cost and performance competitive in the production of semiconductor devices has caused continually increasing device density in integrated circuits. To facilitate the increase in device density, new technologies are constantly needed to allow the feature size of these semiconductor devices to be reduced.
Conventional DRAM cells, whereby DRAM stand for Dynamic Random Access Memory, consist of a transistor and a capacitor mostly made from Silicon dioxide (SiO2). They need the transistor to control the inflow and outflow of charge stored in the capacitor as the physical quantity exploitable for storing information. Said transistor also decouples the capacitors from each other. Such DRAM cells have the disadvantage, that information stored therein is volatile and as such can principally be lost on each power supply failure. Further, the time needed to refresh the information contained in DRAM cells delimits the read and write performance of such cells. Finally, the structure of such a DRAM cell is quite complex due to the required transistor.
Thus, a change in computer RAM technology beyond conventional DRAM technology would be desirable.
The use of ferroelectric non-volatile RAM (NVRAM) cells would already be a great step forward as information would not be lost on any power failure although the structure of the memory cell would remain complex, too. In such ferroelectric RAMs the polarization of the bit storing layer is exploited instead of a capacitor""s capacity in DRAM cells for defining two different states which can be associated with two different logical values. A long term repetitive switching between two different states of remanent polarization, however, fatigues the ferroelectric properties of the material, as e.g. lead zirconium lot titanate (PZT).
In xe2x80x98Physics Todayxe2x80x99 July 1998, page 24 a further high permittivity material and a respective semiconductor fabricating technology is proposed which allows the computer industry to use the equipment of its conventional DRAM manufacturing plants without having to perform basic retooling. It is the so-called high permittivity DRAM technology.
Herein, the charge of a capacitor can be used to store information as it is done in conventional DRAM technology as the polarization of a high permittivity layer depends linearly on the applied voltage, as required for charging the DRAM capacitors. A high permittivity material as e.g. barium strontium titanate (BST) having a permittivity er about 500 instead of er about 4 for silicon dioxide would allow to reduce the space needed for the capacitor as its capacitance is proportional to its area and the magnitude of its permittivity value. This in turn would allow higher integration levels compared to conventional silicon oxide materials used in DRAM cells as the capacitor""s area consumption is large as compared to that of the coupled transistor.
But, nevertheless, as a disadvantage remains that the leakage current is still significant.
Thus, refreshing is a must.
Investigations in the sixties at oxide diodes and thin oxide films revealed several phenomena. For example, J. F. Gibbons and W. E. Beadle reports in their article xe2x80x9cSwitching properties of thin NiO filmsxe2x80x9d, Solid-State Electronics, Pergamon Press 1964, Vol. 7, pp. 785-797, about a two-terminal solid-state switch made from a thin film of nickel oxide. After about 100-1000 switching cycles, the device could not be switched out of an ON condition with normal switching signal amplitudes. Other tests on oxide diodes were performed whereby switching was induced by applying high voltages. These diodes broke down after a few cycles and became unusable. T. W. Hickmott reports in Applied Physics Letters, Vol. 6, No. 6, on page 106 and in the Journal of Vacuum Science and Technology, Vol. 6, No. 5 on page 828 about bistable switching in Niobium oxide diodes. He noticed that the metal electrode plays an important role. To sum up, the tested devices and materials showed that they were either difficult to control or unreliable.
In U.S. Pat. No. 4,931,763 a memory switch is described that bases on metal oxide thin films. The memory switch is irreversible and therefore only switches once. It can be used as connection element in circuits and arrays but not for storing of changing information.
Finally, with increasing integration near and beyond the 1 Gbit chip due to smaller capacitor size the area consumption of the transistor of a memory cell is not negligible anymore. Thus, a great step forward to Ultra Large Scale Integration (ULSI) would be to simplify the structure of a memory cell as much as possible.
Therefore, an object of the present invention is to provide a robust simply structured, reliable and non-volatile memory cell.
It is another object of the present invention to provide a new simpler method for stable storage of information into such a memory cell and a reproducible erasing and reading from it.
It is another object of the present invention to provide a simply structured and non-volatile memory cell which is able to store more than only two distinct values, i.e., which is usable for multilevel storage.
These objects of the invention are achieved by the features stated in the following description and claims.
The basic discovery underlying the present invention concerns a plurality of doped oxide substances including perovskites and related compounds, i.e. materials, for use in microelectronic devices and in electronic circuits and particularly for use in semiconductor chips which combine both, a switching phenomenon in resistance and a built-in memory.
A feature of the present invention includes a microelectronic device design such that it comprises a region between electrodes having a switchable ohmic resistance wherein the region is made of a substance comprising components Ax, By, and oxygen Oz. The ohmic resistance in the region is reversibly switchable between different states by applying different voltage pulses. The different voltage pulses lead to the respective different states. An appropriate amount of dopant(s) in the substance improves the switching, whereby the microelectronic device becomes controllable and reliable.
In general and coinciding with the wording of the claims substances are meant comprising components A x, B y, and oxygen Oz, in which substance said component A is a member of Alkaline metals (group IA in the periodic system of elements), or Alkaline Earth metals (group IIA), or Rare Earth elements, or Scandium, or Yttrium, said component B is a transition metal being member of one of the groups IB to VIII, or a member of one of the groups IIIA, IVA, VA and the substance has a crystalline structure.
Various other objects, features, and attendant advantages of the present invention will become more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the several views.